A retired FDNY captain wanted to show some tough love to his troubled son by not bailing him out of jail for a theft on Long Island — but was devastated when the son died in police custody, family members said Tuesday.

John P. Gleeson, 40, was jailed on May 22 for stealing a manhole cover from a parking lot in Oceanside. He was charged with petit larceny and sent to the Nassau County Jail.

“He was down on his luck, trying to get scrap metal from abandoned buildings, whatever he could get his hands on,” the distraught dad explained.

His son told guards on July 14 that he had swelling and pain spreading from his hands to his chest and neck, the elder Gleeson said.

The inmate had explained the problem stemmed from a medical condition that is life-threatening when it reaches his chest and neck.

Several hours later, he passed out and was rushed to Nassau County Medical Center, where he died.

A correction officer called the family home and the victim’s mother, Margaret, answered.

“Are you John Gleeson’s mother?” the voice on the other end of the phone asked. “Yes,” the mother replied. “Well, he expired,” the retired firefighter recalled.

“My wife started to scream and he [the man on the phone] said, ‘You want more information, call the Nassau County Medical Center,’ and he hung up,” John Sr. said.

A friend of a friend who works at the jail told the Gleesons of their only son’s last agonizing hours.

The Nassau County Sheriff’s Department did not return calls seeking comment.

“They didn’t do anything,” John Sr. said. “They gave him Benadryl that was doing nothing and he kept complaining.”

The family knew something was wrong at 6:30 p.m., the time the inmate would always call his mom.

“He didn’t call because he couldn’t talk,” the father said. “They left him there until 10:30 and took him to the hospital after he passed out. He was declared dead there a half-hour later. I buried him on Saturday.”

Despite a struggle with opiate addiction, John Jr.’s love and devotion for his children never wavered, his family said. He had a 10-year-old son, John, and a 12-year-old daughter, Erin.

“He was an excellent father, ­believe it or not,” John Sr. said while choking back tears. “They’re devastated.”

Last year, Gleeson Jr. was arrested for forging a prescription for oxycodone.